Recently, cellular phones are developing in various aspects for attaining a small thickness and compactness, high performance and the like. Due to the requirements for a small thickness and cost reduction of such cellular phones, there is an increasing trend in employment of a bare type vibration portion for reducing the number of components and attaining a small thickness. In general, an exhaust chamber made of a mold or the like in which sound is rectified is used for vibration control in producing sound of a speaker, or alternatively, a speaker is mounted on a flat portion for attaining stable vibration to realize uniform sound radiation, but in such a case, it is necessary to provide a prescribed space within the device. Therefore, it is extremely difficult, in pursuing compactness, for a speaker vibration portion to attain stable vibration and uniform sound radiation.
In one of audio units used in various electronic equipment such as cellular phones and information terminal devices, a frame-shaped spacer is adhered onto the outer circumference on the top face of a casing and a flexible sheet is adhered onto the top face of the spacer, so as to form a prescribed closed space between the casing and the sheet, and sound of a speaker is transmitted to the closed space (see, for example, Patent Document 1).
Alternatively, in a key switch structure precedently filed by the present Applicant, a metal dome for attaining electric conductiveness when pressed is disposed correspondingly to each switch contact on a switch substrate, and the metal dome is fixed on the switch substrate with an adhesive PET sheet (see, for example, Patent Document 2).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-40621
Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-317386